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Does wing sweep affect the direction of lift?
Given both a straight and swept wing (identical otherwise), exposed to the same airflow, would the direction of lift be the same? Please see below a crudely drawn example of my thinking.

Additionally, as angle of attack is defined as the angle between the airfoil's chord line and the direction of oncoming flow, is there a name given for the angle between the span line and direction of oncoming flow?

The Lift on an aircraft is the vector sum of one component of the aerodynamic forces pressing on the surface of the aircraft as they are distributed over the surface of the aircraft. At each and every point of the aircraft surface (top, bottom, sides, everywhere) the air is pressing on the surface normal, (perpendicular) to the 2-D surface. The vector sum of all these forces is the total aerodynamic force (**one component of which is lift). If the aircraft is laterally symmetrical, then of course the total vector sum will be straight up (as any lateral component from one side of the aircraft should more or less be counteracted by an equal but opposite lateral force from the other side.
The individual forces (acting on each point of the surface, including the wing), if pictured as a distribution of force over the surface, are always normal to or perpendicular to the surface. So if the wing has dihedral, then they are indeed angled in towards the fuselage. But in your "perhaps?" illustration, with a wing with zero dihedral, they would all be pointing straight up.
** The Lift component is the component of this aerodynamic force which is perpendicular to the flight path of the aircraft, the Drag component is the component parallel to the flight path.

Considering that the lift, together with drag and side forces are the resultant of the distributed air pressure on the surface, their definition highly depends on the axis system used. For example, the lift is defined as projection of the resultant force of the distributed pressure into the plane of symmetry, perpendicular to the flow direction. So, the lift direction is defined by the symmetry plane and flow direction, and change of lift direction with sweep, dihedral, or other wing parameters are meaningless. But, it should be said how the pressure distribution changes with the variation of wing parameters.
Regarding your second question, I have never heard something about span line, but if locking in the front view, the angle between the span and the horizon is called the bank angle. If you look from up-view, the angle between the fuselage and air flow is called side-slip angle (considering AOA=0).


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I am an author doing research.
I need information on the following aspects of flying a small plane.
1). What is involved in a small craft pre-flight inspection?
2). When do you need to, and what is involved in filing a flight plan for a small craft?
3). What things are looked at most closely during a small craft crash investigation? Is there still a 'black box' on a small craft?
Any assistance would be greatly appreciated.

You should consider splitting this into multiple questions. You are asking three questions at once here.

"I am an author doing research." That's a statement, not a question, and we're used to seeing questions here as the title, and as @Jimy said, it's best to ask discrete questions. Also be as specific as possible. For example, what do you meant by small craft? Are you talking about single-engine 2 to 6 seats, or like small commuter aircraft up to 19 seats (there's a U.S. regulatory break there). If you're talking about the first, to answer one question, they have never had black boxes. Also, which jurisdiction are you asking about, i.e U.S, European Union, or other.

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How high can oil pressure go on a 737 MAX?
I'm attempting to simulate the EICAS page on the 737 MAX MFD, and I'm researching some limits for the engine based on the FCOM. I've been able to find information at the 737 Tech Site on the lower operating limits for oil pressure, but I didn't find anything talking about how high the pressure can go. I imagine this number should exist somehow, so what would it be? Thanks!

About 41000ft, I'd guess

The 737 does not have an EICAS page, but does have an engine display.  E.g. https://aviation.stackexchange.com/questions/74224/what-does-vib-stand-for-in-eicas  Websites which say associate a 737 with EICAS are simply wrong. Are you sure you’re looking at an 737 FCOM?

According to the type certificate data sheet https://www.easa.europa.eu/en/document-library/type-certificates/engine-cs-e/easae115-leap-1b-series-engines there is a minimum oil pressure listed, but no maximum oil pressure listed. Therefore I assume that there is no limit, warning or alarm for too high oil pressure.

@tedioustortoise I think I got the word "EICAS" from the 737 tech site, but my FCOM does not refer to it that way.

It's a Boeing, there is no limit.

There is an upper limit for safe operation that should be mentioned in the FCOM but the highest figure that can be reported is the highest reading the sensor(s) can generate. Finding that would require getting access to the component specifications which are not (AFAIK) in the public domain.

The question is a bit vague. Which pressure exactly would like to know?

The real maximum pressure in normal conditions
The real maximum pressure before some line breaks (if the safety over-pressure valve doesn't work?)
The maximum pressure read by the sensor
The maximum pressure displayed by the screen (which depends on how the EEC interprets the sensor values - it may discard too high values)

1-2-3 are difficult to obtain because only internal Boeing documents have these information. However, if your plan is to simulate the display, you can roughly estimate 4 by just interpolating a real image on the display (assuming linear the graphic). For example:

For 61psi (green line) you have 30px from the yellow line in this image, 27px left, thus doing the propotion you get something around 115-120psi at the top blue line. If you find other images (with better quality) you can redo the calculus and improve the estimation (I assume it should be a "round" number, 115 or 120 are good candidates).

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How to determine QNH zones like New Zealand did?
I saw a file stating that New Zealand has 11 zones and was wondering how it was determined and divided, does anyone have any idea?
the paper offers a short description that doesn't cover much information,
So if anyone have any information that could help me and guide me that would be great.
Thank you

There's no big secret here. Take meteorological data from whatever sources you have available and plot the sea-level pressure across the country over time. Then use those results to find the areas that tend to have similar sea-level pressure. What more do you want?

I suspect the purpose of the zones are being misunderstood.  The zones do not represent geological areas that tend to share common pressures distinct from the other areas.
They’re atmospherically arbitrary boundaries that ensure that all aircraft in a defined airspace area are operating on the same altimeter settings.  All aircraft in Otago use one identical setting, and an aircraft coming from Canterbury resets its altimeter to the Otago setting at the designated boundary.
This would be useful in any geography where the altimeter settings between two nearby airports could be different enough to cause altitude conflicts if two aircraft are each using a different airport’s settings.

The reference to the AIP has most of the information you're after (AIP ENR 1.7-2). Interestingly there seems to now be 12 zones.
Screenshot from my phone:
Presumably, they created this after studying the historical QNH values and found they were similar within those areas. Pilots can use the zone QNH until they are within the vicinity of an aerodrome, when they switch to the aerodrome QNH.

Thank you for the answer Ben!Would you mind sharing the link?

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Remotely controlled civilian passenger jets
Remotely controlled aircraft have existed since at least the first "world war" (eg, The Aerial Target), so any sensible review would suggest that all civilian passenger fleets would have been equipped with such systems since their inception. Are civilian passenger jets equipped with systems that allow remote conscription and control by the US military? If so, how long have these systems existed?

Comments are not for extended discussion; this conversation has been moved to chat.

No, civilian airplanes that are designed to be piloted don't have any facility for remote control, and remote-control drones don't have on-board pilot controls. The only exceptions to this are prototypes and one-off refits.
For instance, the Boeing 720 that was used to test a fuel additive that was supposed to reduce the risk of a post-crash fire had remote-control hardware added to it, but also carried a pilot during test runs. But the military had no involvement in this, the remote control was done by a NASA test pilot.
Another example is the Wisk Air Taxi, a prototype for a proposed fully-autonomous aircraft. According to their proposal, if these aircraft enter service, Wisk would keep a staff of licensed pilots on hand at their office to take remote control if one of them experiences a situation that the automated flight software can't handle. Again, though, there's no military involvement at all, any remote flying would be done by a Wisk employee.

Comments are not for extended discussion; this conversation has been moved to chat.

In some sense, all fly-by-wire aircraft are "remote controlled". The person with the controller just happens to sit inside the aircraft. There's even research going on to replace cabling with wireless transmission to reduce weight. Then the cockpit would literally become a wireless controller.

@JörgWMittag, why stop at fly-by-wire?  Why not consider hydraulically actuated surfaces as remote controlled?  Or pushrods, or cables...?  "Remote controlled" by definition means the controller is not inside the aircraft - I'd suggest we leave it at that, otherwise it just gets silly.


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Why close isolation valve and APU bleed air for engine fire?
For engine fire, the reason to close the packs (affected side) is to prevent smoke from filling up the cabin? But why the Isolation Valve and APU Bleed Air need to be closed too?

Might be to stop the flame propagation to the APU, through the bleed duct. So, closing the bleed would essentially stop the flame propagation to the APU.

Closing the isolation & APU bleed valves keeps hot bleed air away from areas near the engine that may have damaged bleed air ducting. Closing the Pack valve causes the other pack to switch to high flow, making up for some of the lost output of the affected-side's pack. The steps are together in the checklist because it's all on the same overhead panel, even though the reasons differ.

Closing the pack valve would also provide some additional containment of smoke, fumes, and flames if the isolation valve fails.

I don't fly B737 aircraft.  I would imagine the APU bleed air is used to start the engine.  The other way is to probably use a huffer cart.
If there is an engine fire, that smoke can travel the same path back to the APU and then that smoke can be brought into the cabin through the other pack.  So the idea, is to prevent any smoke from entering the cabin either directly from the engine or indirectly through the APU.

If the APU bleed valve is open, it's so that the running APU can supply bleed air. The bleed air FROM the APU would have far more pressure than smoke merely "in" the duct. In fact the bleed air from the APU would clear smoke out of the bleed duct pretty quickly. No, the issue is avoiding the (smoke-free but very hot) bleed air from the APU getting to places that have been possibly damaged by the engine fire.


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Why do fighter jets pilots use an oxygen mask?
I always wondered why fighter pilots need to fly with a facemask.
Since the cabin in pressurised wouldn't the oxygen level be the same as in any commercial jet so sufficient for the pilot to remain conscious and aware?
Thank you

Does this answer your question? Are fighter jet cabins pressurized?

In addition to @TooTea's linked answer, fighter pilots are meant to operate in environments where other fighter pilots try to make holes in each other's aircraft. This could result in no pressure in the cockpit, which would be sad.

Plus, the oxygen level (percentage based on volume) is just one part of the equation. Oxygen has to be supplied at or above a certain pressure for your lungs and blood cells to absorb it. At sea level pressure, your entire oxygen absorption and transport mechanism would work fine (barring physiological or chemical problems). At 30,000 feet, even 100% oxygen would have a hard time absorbing. It is easier to pressurize a mask versus an entire cockpit in an emergency.

the pressurization system in a fighter plane might be able to furnish 10,000 ft cabin altitude at 30,000 feet, but the pilot would need supplemental oxygen when flying higher than that. In addition, if the cabin is breached above 30,000 ft the pilot would have very little time to get his or her mask on before blacking out, so the mask is worn at all times.

Exactly. To add to that, I believe the quoted time of useful consciousness at 30,000 feet is under 60 seconds. I am going off of memory for that stat. But, I believe it is correct.

In that case why bothering with a pressurisation system at all and not saving space/weight engineering studies etc and only giving the pilot a facemask?

because without pressurization, even breathing 100% oxygen above 20,000 feet is insufficient to keep the pilot awake and alive.


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Aircraft Specific Type Rating Check-ride vs. Airline Transport Pilot (ATP) Check-ride
When looking at the Airman Certification Standards (ACS) for “Airline Transport Pilot and Type Rating for Airplane” I have interpreted the level of precision (e.g., hdg. +/- 10°) on a type rating (e.g., B737, A320, etc.) check-ride to be the exact same precision required when an applicant is testing for an addition of an ATP license (i.e., check-ride). For example, in the ATP & TR for Airplane ACS for normal take-off, the pilot must maintain climb airspeed +/-5 knots. For a Type Rating check-ride, I gather the same precision (i.e., +/-5 knots) is required since the document title includes "Type Ratings"?

Is there a particular reason that would cause you to question this? Otherwise I don't see any reason to doubt that your interpretation is correct.

No, just the general rule that nothing in dealing with the regulatory authorities is ever as straightforward as it may at first appear.

I don't know if there is a question posed by the OP.  The level of precision flying required of a type rating applicant is the same as what is required if applying for an ATP certificate.
The FAA, rightfully so, considers anyone who flies a type rated airplane to need to have the necessary piloting skill, CRM, ADM and leadership skills required of any ATP certificate holder.
The oral portion of the practical test for the type rating versus an ATP certificate is almost identical with some exceptions.  An ATP applicant will need to answer questions they were found deficient on during the written test as well as an additional 5 subject areas.

Task I.C. Weather Information
Task I.D. High Altitude Aerodynamics
Task I.E. Air Carrier Operations
Task I.F. Human Factors
Task I.G. The Code of Federal Regulations.

There will be no difference in the flying portion of the practical test between a type rating and an ATP certificate.


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Flight Deck Visibility Reference Source

Where does this drawing (T-tail airplane on approach) come from? I have seen it cited before in conversations but without a reference such as an advisory circular or AMC.

Is there a higher-resolution version?  That would let us use a font identifier, which might narrow down the decade or country.  2. Both meters and feet is unusual;  what other diagrams have that?

It actually comes from the "Toren book", that is Synthesis of subsonic airplane design (1976) by Egbert Torenbeek, an engineer and professor of Aerospace Engineering at Delft University of Technology:
On page 92:

This diagram illustrates a proposal to change the regulation about cockpit design, e.g. FAR 25.777: standards must be evolved for this category of aircraft to ensure clear areas of vision during the approach. [...] The angle of view forward and downward must be sufficient to allow the pilot to see the approach and/or touchdown zone lights over a distance equal to the distance covered
in 3 seconds at the landing speed when the aircraft is [...] at a decision height which places the lowest part of the aircraft at a height of 100 feet above the touchdown zone.
You can access a scan of 2nd edition at TU Delft, be patient, the file size is large and the server datarate is limited.

This image has been used once on this stack: Why does the Boeing 787 have fewer cockpit windows?

Also was used in this thread. Nice find!


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How to find TAS, IAS, PPH and USG?
Could somebody explain this task. I do not understand how to use this table :(

It's been 40 years since I did one of these, but here is how I would proceed (and hoping someone else will correct this if I've got it wrong). Let's start the first one: 25" @2500 rpm FL90 ISA+5°.
First, 25" is the first line of each of the 3 ISA deviations of the table: -20, 0, and +20. So, so you're going to have to get an answer for 0° and +20° and then interpolate between the two to get +5°.
Let's get the answer for 0°, which means we won't use the 25" line but rather use FL90, which is 9000 feet. In the U.S. we don't (or at least didn't) use FLs, Flight Levels, below 18000, but in other countries they do. There is no 9000 feet entry, so you're going to have to get the answer for 8000 and 10000 and interpolate between the two.
So there are a set of answers for 8000, which you will have to interpolate between those and the set for 10000.
And then you'll have to get the answers for +20° and then do that interpolation.
Lazy man that I am, the only time I actually used these kind of tables accurately (or at least attempted to do so) was in taking exams. In actual practice I never did. Ball-parking the answers was always sufficient until I got to the airlines, and then all that was taken care of by the dispatchers.
So, for the exams, work several of these (in spite of their frustrating, time consuming nature), and develop an approach to doing them. Your most likely errors may well be in interpolating, which is why one approach in real life usage is to simply not interpolate but use the most conservative of the two values you would otherwise have interpolated between.
Really, the important thing is to not run out of fuel, which is why some might have called me "Tanker Terry."

Thank you so much!! :)


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Gulfstream IV-SP Not Sending POS INIT to IRS 3
After powering up and starting up EFIS/ICAS, starting up both FMS’s, I try to POS INIT the IRS’s, but the IRS in position 3 does not receive a position. It’s like it’s on and waiting for it, but it only sends the position to IRS 1 and 2. Has anyone had this issue before?

Is this on a real aircraft or a payware flight simulator thing?


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How many active registered Cessna 172s are there in each United States state
This is for a school project on different aircraft accidents. (My sources I have been given are the FAA and the NTSB.

Welcome to Av.SE. You're more likely to get good answers if you demonstrate how you've attempted to find some solution yourself. Have you dived into the websites for those 2 agencies? Tried specific search terms on the internet? Looked at the manufacturer's website or a forum for owners of these aircraft? If you're part way there, someone here may be able to offer advice to get you the rest of the way, but not so many are likely to be interested in turning your research project into their research project.

How many active registered Cessna 172s are there in each United States state

You can use the FAA registry. E.g. to get all 172:

Go to this inquiry page
Enter Cessna and 172, press `Submit.
Get the Excel file and sum all variants.

Enter 172S if you want to have only the 172S variant, etc.


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Which is more efficient: a jet engine or a rocket-powered engine?
Knowing that rocket powered engines have higher speeds than jet engines, it raises the question: are jet engines more efficient than rocket powered engines?
This is of course in terms of fuel consumption per distance traveled, as I am curious whether a rocket engine allows an airplane to travel a distance more quickly compared to a jet engine when consuming the same amount of fuel.

The only chance a rocket has is a suborbital flight.  Get ICBM data and compare it to a 747.  The jet will have an advantage in fuel consumption per distance, the rocket much faster.

No, rockets engines donnot have higher speed than jet engines. That one was not designed to exceed mach1 while that one easily exceed mach1 without using full power.

@ManuH Super's formulation is a bit inexact, but it's not wrong. The highest speed attainable by a jet engine is in the region of Mach 3.5. Rockets can reach much higher speeds than that.

You should be more clear in the title that you are asking about jet powered airplane with rocket powered ballistic transport vehicle.

@Hobbes I agree, and that's I think the formulation must be clarified. As is, it is unclear if it is an false assumption, a mistake, or something forgotten (or something else)

Jet engines are far more efficient. The metric to compare them is 'Thrust-specific fuel consumption', i.e. the amount of fuel used to produce one unit of thrust. Fuel consumption per distance traveled is proportional to this.
A few figures from that page:
Rocket engine: Space Shuttle main engine, 225 g/kN.s
Jet engine in afterburner: EJ-200: 48 g/kN.s
Jet engine, dry: CF-6: 17 g/kN.s
So a rocket uses ~10 times as much fuel to produce the same thrust. Even if you add afterburning, a jet is more efficient.
During and after World War 2, there was some interest in rocket-engined aircraft. The Me-163 saw service in the Luftwaffe.
The interest continued for a decade or so, until jet engines with afterburners were developed. One exponent of that interest was the Saunders-Roe SR-53, a short-range interceptor with both a jet engine and a rocket.
It was developed in response to an RAF requirement for an aircraft that could take off and get to high altitudes very quickly; more quickly than could be attained with the jet engines of the day.

Jet: Armstrong Siddeley Viper, 1640 lb thrust.
Rocket: De Havilland Spectre, 8000 lb thrust.
Fuel carried:
jet: 500 lbs
rocket: 10,500 lbs
Endurance:
jet: 45 minutes
rocket: up to 7 minutes
speed:
jet: 180 kts
rocket: accelerate to Mach 2 once, then the fuel ran out
(source: mostly 'British experimental jet aircraft', Barrie Hygate.)
It was superseded by the EE. Lightning, which had 2 afterburning jets. It had 4 times the range despite carrying less fuel and more payload.

Arguably an afterburner is actually a kind of rocket, which rather muddies the water.

A rocket carries its own oxidizer, an afterburning jet doesn't.

True, but now define a Jet engine in such simple terms. I would define a rocket as a device that produces thrust purely by burning fuel. Of course that interferes with the most obvious exception to your definition - not all rockets carry oxidiser as such.

Question defines efficiency as fuel per unit payload per unit distance. This doesn't answer the question.

@user3528438 Yes, it does. Replace the jet engines on an aircraft with rockets of the same thrust, and fuel consumption (for traveling the same distance at the same speed) will increase by a factor of 10.

Have you included oxidiser in the weight? It seems like you have included oxidiser in the weight for rockets but not for jets?

Yes, I included oxidiser in the weight for the rocket, and not for the jet. Because the rocket has to carry its own oxidiser, which means you have to provide space and weight allowance for it in your rocket/aircraft. You cold split it out to fuel and oxidiser to improve the answer, that requires knowing the fuel/oxidiser ratio of the rocket engine.

@Hobbes: Even more important, a rocket has to carry all its own reaction mass.  An airbreathing jet engine gets huge amounts of reaction mass from the atmosphere for free (even if the engine used all the oxygen in its intake air for combustion - and even an afterburning turbojet uses up only a small fraction of said oxygen, since it has to run very lean in order to avoid consuming itself - the four-fifths of the air that's made up of nitrogen and argon still serves as dedicated reaction mass).

The key to why an air-breathing jet engine is "more efficient" is that it doesn't need to carry oxidizer, it can just use the atmosphere. Carrying oxidizer is very heavy, which will severely impact the range and speed of the aircraft.

Also, an airbreathing jet engine gets free reaction mass from the atmosphere (the vast majority of the air that goes into a jet engine serves as reaction mass, with only a tiny fraction being used as oxidizer), whereas a rocket's supply of reaction mass is limited to its onboard fuel and oxidizer.

"Ballpark" figures for an 8000 mile trip Atlas ICBM vs 747 jet airliner:
Atlas ICBM:  8000 lbs delivered using 75 tons of RP-1/225 tons LOX = 27 lbs/ton total propellant, or 108 lbs/ton on a hydrocarbon fuel basis.
747 airliner:  150,000 lbs passengers and luggage using 150 tons fuel = 1000 lbs/ton
                        and a softer landing.
Biggest disadvantage of the rocket is having to carry it own oxidizer, as 1 lb kerosene requires more than 3 lbs of oxygen for combustion.  Not using a wing also greatly reduces efficiency.  The winged jet is around 40 times more efficient for this application.


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Do smaller wings mean faster flight?
I've known that military aircraft with small wings can fly very fast, while commercial aircraft with big wings cannot fly as fast. Why is this?

Related: Why do some light sport aircraft have clipped wings?

"Small wing" or "big wing" is a vague term. Concorde's wing area is almost 2 times larger than Boeing 757, but it could fly 2.5 times faster :)

@ElectricPilot I'm guessing the OP means the ratio of wing area to aircraft length.

On the whole, smaller wings produce less drag, but they also produce less lift. Less drag means the same engine can reach a faster cruise speed, but less lift limits the weight capacity and means the aircraft needs to go faster to stay airborne: the stall speed, take-off speed, and landing speed will all be faster, which in turn means longer runways are needed (or systems like catapults and landing chutes).
For a fighter aircraft, that top speed matters a lot more than the lifting capacity or take-off speed, so they're optimised with these low-drag, high-lift wings. Compare to a utility aircraft of the same size: the utility aircraft will have a less powerful engine, and wings that produce more lift at lower speeds. A utility aircraft that needed the same kind of runway as a fighter would be pretty useless.
In the other direction, compare to an airliner. These wings are tuned like a fighter's: to be maximally efficient in the cruise. But the airliner has to lift a lot more weight, so the whole aircraft is much larger, and the wings are in proportion to that. They typically also require long runways, and fly faster than a small utility aircraft. But look at the Hercules military transport. It's designed to lift a lot, but it also needs to operate from poor-condition, short runways. It has less efficient, high-lift wings, so its take-off speed is very slow for its size, and it isn't that fast in the cruise.
In summary, wings (not just wingspan, but also the shape of the wing) allow you to trade off lift and drag. High lift and drag gives you slower take-off and landing speeds, but you pay for that with a less efficient cruise. Low lift and drag gives you a faster cruise, but you pay for that with ridiculous take-off and landing speeds and the extra runway that requires.

Bottom line: It all comes down to what the aircraft is used for. Airliners simply need to maximize efficiency over speed, and jet fighters need the luxury of speed to ensure competence in combat applications.
You also asked about the relationship between speed and the wings, so let's get to that. This is a broad question, as some civilian planes have quite small wings relative to their size, and military planes with large ones, also relatively. However I will try to answer this to the best I can: There are a few major factors that affect the wing area-speed relationship, but it almost always boils down to the purpose of the plane itself (which is the determining factor usually/hopefully!). Let's start with purpose of design.
The large commercial aircraft you refer to in your question, i.e. most of Boeing and Airbus' product line tend to have larger wings simply because as they are much heavier as they get larger which requires greater lift. The usual swept wings found on these aircraft are optimized for maximum efficiency in subsonic flight. There's been a lot of study into improving this, but for now the basic narrow/wide body airliner with a cylindrical fuselage and conventional wing arrangement is the easiest to produce, and with this the swept wing design is the most efficient at the most fuel efficient speed for the design (about 0.80-0.90 Mach). It's a fact of life and physics, not much else to say unless you want to delve into the aerodynamics of it (please if you want me to say so below I'd be more than willing). In addition, these aircraft aren't required to execute fancy maneuvers, nor do they need to be resistant to battle damage or carry munitions underwing, which has allowed designers to trim the airfoils quite a bit. Sure the engines weigh quite a bit, but unlike bombs and missiles they also provide thrust and some lift, and they don't or aren't supposed to fall off I hope. Now onto the military application.
I assume you are asking about jet fighters, of the air to air variety, such as the Su-27 and F-22. I can see why you think that these planes' wings are small, but actually comparing their wing to body ratio will reveal that it's not much smaller and sometimes greater than that of the airliner. However, you are correct in thinking that the purpose is different, and obviously the wing would be designed to reflect that different purpose. Mainly, jet fighters operate at a FAAR greater range of speeds than airliners, and much more routinely surpass speed barriers that introduce significant aerodynamic challenges. The sound barrier isn't the only one, but it's probably the easiest to explain so I will use this as an example. Subsonic air (airflow slower than the speed of sound at altitude) acts in a different manner than supersonic air, introducing wave shock, and exaggerating problems that can arise due to stagnant flow, etc. In addition to utilizing several mechanisms to avoid these issues such as vortex generators and intake splitters, the design of the wings themselves must be modified. To eliminate problems that increase in severity as you approach Mach 1, including destructive vibration, Mach tuck, and tens of other issues, the wing (as well as the rest of the plane) has to be shaped to increase its critical Mach number, a value that pretty much describes the behaviour of the airflow around the airfoil or wing. Otherwise you will encounter stupid large amounts of drag and more often than not stall out due to an absence of airflow over control surfaces. Again if you want me to delve more into the physics of this let me know, I just am short on time right now. The other factor going into the smaller/different wing is wing loading and maintenance, two smaller factors that I feel are more related than not. In short, the fighter's wing will undergo a lot more G's and other strain in different directions due to the much larger flight envelope (the types and numbers of maneuvers that the aircraft can and has to perform) which necessitates a different type of wing.
How we actually see this disparity in role reflected in aircraft can be through a more swept wing, delta wings (best for supersonic flight), variable-geometry wings (F-14), or even completely straight wings for maneuverability at slower speeds (A-10). I suppose it's best to say that there are a whole many more types of roles a military aircraft can fill, compared to short or long distance hauls for civilian aircraft, and this reflects the sometimes smaller wing sizes found on military specimens.
[Edit] Some military aircraft have more than twice the wing to body ratio, such as the U-2/TR-1 spy plane that airliners possess. Just goes to show that the military requires a greater range of designs to fulfill a greater amount of applications. Performance isn't limited to speed or maneuverability.


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Is it possible to play music in the cockpit?
I was wondering that if a pilot's flight had been delayed or a pilot had nothing to do, would it be possible for a pilot to play music in the cockpit?

Related

Are you talking about commercial pilots or GA? My audio panel has a music input, and even a phone input.

Possible duplicate of Is it allowed to listen to music while piloting a plane?

This question asks, and the answers address, is it possible, while the suggested dupe discusses, is it allowed. Not really the same question, nor quite the same answers. Voting to leave open.

@qqjkztd, Well, even a more advanced music making has been done this way :)

With the proper equipment, it is possible.
For example on the Garmin GTN 750. Music is listed as a service on this avionic.

And you can even create multiple music channels and assign it to different people on the intercom, of course with mute feature when someone speaks either on the intercom or radio.

Depending on the equipment in the aircraft, the answer may very well be yes. A lot of modern headsets in civilian aircraft (particularly those with active noise reduction) have bluetooth capability that allows music streaming. Some more modern audio panels even have music inputs now. I even flew in an Archer that had a CD player on the panel and wired into the audio panel.
If you are lucky enough to have an automatic direction finder (ADF) eating up useful load in your aircraft you may be able to tune into AM radio stations. This may have contributed to the crash of Varig Flight 254.
While you are legally allowed to do this (at least in small planes) the sterile cockpit rules that most if not all air carriers have prevent professional pilots from doing so, as @Carlo Felicione points out in the comments

Good post, though I might add some information in there about sterile cockpit rules for professional pilots .

The politics of pilot distractions has changed in the last 15 years so things may have changed. But 15years ago Atlas freighter pilots were allowed (and somewhat encouraged) to use laptops for movies and iPods for music while in the cockpit to help stave off boredom on long flights. The reasoning was that kept their brain alert and awake and increased safety.


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Is it theoretically possible to create an aircraft like this?
I was browsing through some funny airplane pictures when I came across this. An airplane with the engines replaced with the cockpit and the cockpit replaced with a huge turbofan.
I was intrigued by this idea, but then it also brought up the question: Is it theoretically possible to create an aircraft like this and be possible for the aircraft to take flight? (as in fly in the air for prolonged time)

Source: YouTube

yes but it wouldnt fly for long LMAO

Given enought thrust, anything can fly

But the question is, is there enough thrust in this case?

Like the  CAPRONI-STIPA

Mig 21 isn't far away from this in concept: pilot sitting on top of a big engine with payload under the wings.

@user3528438: MiG-15 & 17 even more so. ttps://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-15 https://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-17  Or the US-built F-86 https://en.wikipedia.org/wiki/North_American_F-86_Sabre

I think it is possible see the aerodyne of alexander lippish...i dont know if he fly

Look like the clip-air

Do you mean a twin fuselage with one large engine?

@SSumner "We need more thrust!" https://www.youtube.com/watch?v=4r6wsDO-Ik4

Unless you show us what the back end of the plane looks like, this question is unanswerable.  A big fan trying to cram air into the front of the fuselage leads to no thrust if the air (including the portion that is expanded after combustion) has nowhere to exit.  Plus it is unclear if or how the big round yellow thing is attached to the plane.

The idea has been tried. This WWII aircraft is built around the idea very much like yours: engine in the main fuselage, pilots/passengers in the nacelle.
It must be reasons to choose this design. The design requirements were:

One engine.
Crew one or two
Best possible view into to all sides
(reconnaissance aircraft).

For the passenger aircraft as pictured it is probably just not optimal.

This aircraft looks like the boomerang

There have been several planes generally similar to this, some quite successful. Early examples were the Italian Savoia-Marchetti S.55 and S.66 twin-hulled flying boats with a row of engines above the hulls and these were the most numerous.

They were the only ones to carry passengers on either side.
The most famous is surely the German WWII Heinkel He 111Z glider tug. It comprised two He 111 twin-engined bombers joined by a straight centre section with a fifth engine in the middle.

The French Fouga CM.88 Gemeaux engine-test aircraft had twin fuselages and for engine testing its original engines were replaced by a single central engine. It flight-tested several jet and turboprop engines in this way and is probably the only one to have flown with a single central jet, indeed any jet.

The Slovenian Pipistrel Taurus G4 is a more recent engine development platform for electric-powered aircraft. A few others have flown, or tried to, but proved useless.

There inst a jet engine that size, and this is very unfathomable when it comes to reliability, safety, and efficiency. But, if you are asking if we can design a plane with the power plant as the main fuselage/airframe, then probably yes, a very bad one.

The GE9X nacelle is only 10cm smaller than the B737 fuselage...

The problem with very large turbofans is that the fans are difficult to scale. To avoid sonic shockwaves and noise they must turn slowly so that the tips stay subsonic, so they have to be geared down off the turbine. Also making the blades strong enough to resists birdstrike and similar gets harder. Nevertheless Rolls-Royce do now have one under development, called the Ultrafan.


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Did the Soviet Union build any commercial jetliners?
I read about a Wikipedia article regarding the Soviet Union's Tupolev 114 which was comparable to the time's jetliners, and brought up the question: Did the Soviet Union build any jetliners for commercial use (i.e, airlines, firefighting).

I am seriously curious what kind thought process and presumptions could lead to such a question.

And who can forget the TU-144, the world's first and biggest commercial supersonic transport aircraft.

industrial espionage on the concorde..and of a much lower quality!

Of course. Probably more than any other single country except the US.
Tupolev Tu-104, one of the first jetliners. For a short time, when the Comet was grounded, it was the only jet airliner flying in the world.
Tu-124, its development, the world's first jetliner with turbofan engines.
Tu-134, further development; claimed to be the world's most produced twin jet of this class (80-pax).
Tu-154, a trijet, USSR's most popular airliner. (Similar to B727 or Trident).
Tu-204 was developed in the 1980s and is essentially 'Soviet', but never saw service in the USSR.
Tu-144, of course. Although it first flew a few months before Concorde, it had extremely limited commercial (or better say, 'civil') service.
Ilyushin Il-62, a long-range jet, a relatively rare example of a quad-jet with all engines on the back (similar to Vickers VC10).
Il-86, USSR's first commercial widebody, and its later development Il-96.
Yakovlev Yak-40, a small (30-40 pax) regional trijet, the world's first in its class.
Yak-42, the first Soviet jet with high-bypass turbofan; a trijet smaller than Tu-154.
All these jets (except Tu-144 and 204) were in wide use in the USSR, and most were exported at least to the Eastern Bloc countries and beyond. Of them, only Il-96 and Yak-42 remain in limited commercial use, although more are operated by air forces and government bodies.

The question was specifically about jetliners. 90s are not 'Soviet Union'. The only borderline case is Tu-204, which was developed during Soviet times, but never saw service in the USSR. I did miss Tu-144 though.


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How long can a passenger aircraft glide without engine power?
Assuming that the aircraft is cruising at 36000 ft and lost engine power, how long could a passenger aircraft glide?

I‘m sure that’s been answered elsewhere already, but until someone digs out a link for proof I‘d go with an ideal maximum value of 20ft forward for every feet altitude which is good enough for a rough estimate. Real scenarios where this happened (search machine keywords: Gimli glider and Air Transat Lajes) involved substantially shorter glides though.

The average airliner has about a 15:1 glide ratio.  If it is 1 mile high, it can glide about 15 miles.  36,000 ft is about 6 miles high so it should be able to glide about  90 miles. (6 x 15 = 90)

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Do commercial pilots have their own lavatory?
I was wondering if commercial pilots had their own lavatory to the two instead of using a passenger lavatory.

Also, what about when commercial aircraft are converted for cargo, where the cargo area is unpressurized. Then is the pressure bulkhead just moved aft a bit?

@Pheric Unpressuirzed cargo holds don't exist on transport jets. See "Are cargo holds pressurised these days?"

@user71659 What about the Beluga?

@Pheric Not a transport jet by common definition or legally. It can't have passengers either, and bottom section of the aircraft isn't converted, it's unique to the model.

@user71659 good point lol. Didn't reread my question

In an Airbus A320; pilots share the toilet with the passengers. On AirAsia, a flight attendant blocks access to a toilet when the pilot enters the lavatory.


text:
How often is a commercial (airliner) flight flown without autopilot?
When it comes to commercial flights, I am aware that once the aircraft has reached a certain altitude, the autopilot activates and assists the pilots in flying the aircraft. It makes being a pilot on a long flight a less tedious job.
But what is the ratio of total flight hours to autopilot-flown hours during a typical flight?

Neither of those answers respond to his "how often" question directly.  The answer is, on a jet, pretty much every flight.  It's exhausting to have to hand fly a jet at cruise because it takes constant attention on the part of both pilots, one having to monitor the other pretty much full time, to avoid busting an altitude clearance.  With the AP on, only one pilot needs to monitor the AP and the other can do whatever they want, doze, read, go for a dump. And in any case, you need a working AP to fly in RVSM airspace.  If you want to fly with the AP inoperative, you are stuck below 29000 ft.

Roger @ymb1 thanks.

Rephrased question in accordance to ymb1's suggestion.

I don't believe that question answers my main question. I could be wrong, of course.


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Is it possible to celebrate a holiday twice by flying from one point on Earth to another?
It recently came to my attention that the different time zones on Earth could allow for a person to celebrate holidays such as New Years twice by flying from one point to another.
Is it possible to fly quickly enough to celebrate a holiday twice?

I'm voting to close this question as off-topic because it belongs on Travel.SE

I'm voting to close this question as off-topic because appears to be more appropriate for Travel SE.

@ymb1 Only if Concorde could fly now, I really wish to experience the supersonic speed.

Does this answer your question? Are there any airlines that fly a route that has an arrival time earlier than its departure time?

Technically, yes. As I right this, it is half past midnight at London, Heathrow. It is half past eight at New York Kennedy. And, it is half past five at Los Angeles. If it were not Daylight Savings Time, it would be half past seven in New York. On New Years Eve, you would have between 4 and 5 hours to celebrate in London, fly and land, before you started celebrating again In New York. Hardly any time for even a celebratory drink (as a passenger) and kiss. I have come very close to landing before I took off  when traveling from the US to the Asia. So far, I have never made it. I’ve always missed it by an hour or three.
It would be easier to celebrate New Years in somewhere like Lubbock, TX (Central Time, USA), then flying to Roswell or Clovis, NM (Mountain Time, USA).

There are plenty of flights that depart Japan/East Asia after midnight and arrive in the US before midnight on the previous day.
Or there's NZ40, departing Auckland NZ at 0955 on, say, January 1, and arrives in Tahiti at 1555 on December 31. You could party, watch the new year fireworks, sleep, fly and arrive early enough to do it all again.


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How to find the angle of attack?
I'm trying to calculate the angle of attack (AOA) using aircraft some data. From what I understand the AOA is just the angle between the aircraft's forward unit vector (out the nose vector) and the x-component of the velocity vector (vx), as long as both vectors are in the same coordinate frame.
My current plan is to find the aircraft's forward unit vector, and then find the angle between that forward vector and the x-component of the aircraft's velocity vector (vx) using the dot product.
All of my data is in the ECEF frame and consists of the position (x,y,z), velocity components (vx,vy,vz), and the orientation components (psi,theta,phi).
There are two stack exchange answers that I think are on the right track, but they are not using the ECEF frame.
In the first answer, https://stackoverflow.com/a/1568687/12131175 the unit vectors of the aircraft can be found using the rotation matrices, but I think they are using the NED coordinate frame and not ECEF.
In the second answer, https://aviation.stackexchange.com/a/67213/52074 the AOA is found using the arctan function, but vectors are in the aircrafts body frame.
So, my questions are:

Am i on the right track on how to calculate the AOA? Is this the best way?
What are the correct rotation matrices needed to solve for the unit vectors in the ECEF frame?

It seems like there's a lot of different sets of rotation matrices out there for the different orders of rotations (x-y-z vs z-y-z and extrinsic vs intrinsic). Some use different greek letters for the same rotation (i.e. rotation about z-axis, some use φ and some use ψ). I'm not certain which rotation matrices are correct for describing an aircrafts orientation in the ECEF frame.
Thanks in advance!

AOA is actually to the wing chord line, not “out the nose”, so make sure you factor in any angle of incidence built into the airframe...

What's the ECEF frame?

@MichaelHall Thanks.

@JZYL Earth Centered Earth Fixed frame. Also known as ECR. https://en.wikipedia.org/wiki/ECEF

No, you can't find AOA simply by looking at inertial velocity. You need to know winds too (expressed in whichever frame you're doing your calculations in).

@JZYL Understood. I guess that's why the AOA is usually measured using an onboard sensor? I just wanted a theoretical value by assuming the wind vector was in the opposite direction of the velocity vector. I suppose this may be evolving into more of a math/physics question if it's not the actual AOA that I'm calculating since I don't have any wind data.

I think you misunderstand the definition of AoA. It is defined between mean chord (not zero angle with the fuselage) and relative wind (not necessarily constant in time). See skybrary for more information.

@ManuH I think the data is actually for a missile and not a plane, so in that case could I assume the chord line is on the same axis as the out the nose vector? There may be fins, but I assume those are also aligned with the missile's center axis.

It doesn't make sense to do this in ECEF. Convert the speed vector to the body reference frame and calculate the angle between the body x axis and the projection of the speed vector on the xz plane. Add the angle of incidence and you're done. Assuming no wind.

@DeltaLima Do you mean to convert the speed vector to the NED frame? Exactly which frame should I convert it to?

You should convert  everything to the body frame

@DeltaLima Thanks for the info. I'm still confused, doesn't the body frame just move with the object? So, wouldn't it have to be relative to some fixed frame? Do you have any links on how to do what you're referring to? Thanks!

Regarding your first question

The angle of attack is the angle between the chord line of the
airfoil and the relative airflow, that's first and foremost.
There is a longitudinal datum that could be what you are referring to
as the "aircraft's forward unit vector" but that longitudinal datum
shouldn't be measured directly to the velocity vector in your case
because that's not what the air considerably is passing through, the
air passes through the airfoil which has the chord line in it, the
angle between the chord line and the longitudinal datum is called the
angle of incidence and it's a fixed angle.

The velocity vector could be the relative airflow which is a
combination of more than one direction of wind.

Regarding your second question

I think the concentration should be towards the lateral axis of the
airfoil which that airflow is angled toward and rotates on to create
a pitch angle whether it is a positive or a negative angle of attack.

This video about the propeller's angle of attack would help you understand the combination of the velocity vector:

This would explain your second question further (Link is http only 'not secure') : http://www.prepar3d.com/SDKv2/LearningCenter/simobjects/weapon_systems/weapon_attachments.html


text:
How can we adjust the features of an airfoil?
We have a base airfoil with

Max thickness 11.7% at 28% chord
and
Max camber 3.4% at 42% chord.

Say, we want to draw our own airfoil based on the previous description but with an

increase maximum percentage of chord by +3%
and
increase thickness by 0.2%

Which method and software should we use?
Which position coordinates should I edit to achieve this?
I want to draw my own airfoil and run it through CFD.
Thank you in advance.


text:
Have any regulatory exceptions been made to allow a supersonic aircraft to fly over land so that it can follow the path of a solar eclipse?
It seems like there might be a good research reason to allow such a flight. As it's an unusual event, making this exception would not set much of a precedent.
It could potentially be a good way for NASA to assess the performance of the X-59 - if it's ready in time.

Just as a side note, the totality will be moving across the surface much faster than the X-59's planned top speed. It would not be able to keep up (even if it actually were ready for supersonic flight testing 6 weeks from now, which seems unlikely,) though it could experience totality longer than viewers on the ground.

Good point! The moon's umbral shadow speed is 2286 mph. The Earth's rotation at the equator is about 1040 mph. The X-59 is expected to fly at 925 mph.

I guess this can be a question for Eric Célérier who flew a Concorde following the 1999 solar eclipse path.

Interesting article. They estimated they would experience 8 minutes of totality flying in the Concorde. They were also worried about the shockwaves effects when two Concords are flying in close proximity to one another.

There was also a scientific mission for the 1973 solar eclipse over Mauritania and Tchad, using the first Concorde prototype (no restriction existed yet). Traffic in the area was interrupted. The shadow cone was slower than in 1999, 7 min. of totality on the ground. The cone traveled about 200 km/h in excess of Mach 2, allowing 70 min. of totality in fight. More details.

The blanket ban on supersonic flight over the US only applied to civilian aircraft. The military has the ability to design supersonic flight profiles in concert with the FAA, so the X59 idea could be doable, depending on the flight parameters.
Within the past couple of years the FAA has published this rule to allow supersonic test flights of civilian aircraft over the US. The rule is intended to allow industry a pathway from R&D to actual supersonic flight testing. If a company could show the flight as a test, and the profile was acceptable, a civil aircraft may be able to fly the mission you propose.

NASA's aircraft are normally subject to civilian rules, though, aren't they?


text:
Have two similarly sized aircraft ever hard-docked with each other while in flight?
This question is looking for a precedent in aviation related to the futuristic idea of maintaining a permanent fleet of solar-powered aircraft in the upper atmosphere on the sunny side of Venus (which has Earthlike temperatures and pressures and 24/7 strong sunlight). See https://space.stackexchange.com/questions/65119/could-an-acid-proof-solar-powered-aircraft-stay-aloft-on-the-sunny-side-of-venu
While I'm aware of planes flying in a tight formation, in-flight refueling, the idea of one plane carrying another plane, and the idea of a plane towing a glider, this question takes these concepts a bit further.
The application is to hard-dock two planes while in flight so that one of them can be powered down and serviced while the other plane provides thrust, control, and even lift.
Ideally, for this application, "service planes" should be able to service each other as well as other kinds of non-service planes.
Has any aspect of this idea ever been seriously studied or even demonstrated - perhaps for some other similar kind of application in aviation?

Related concept FICON project

If you define aviation to include space craft then yes, the Lunar Module and Command Module of the Apollo era routinely hard docked with each other in flight.
There was also the Apollo Soyuz docking project which brought together two craft from different programs.
If you further extend the idea to include "stations" both the Space Shuttle and the Soyuz docked with the ISS (which is technically a moving object) to refuel and re-supply it regularly. The Apollo system was also used to get to and dock with Skylab

Good point - I did not think of that. While docking is space might be easier in some ways, its still relevant experience.

There was a case when one F-4 have been used to push mechanically another F-4 that was damaged and out of fuel, hence providing thrust (Pardo's push). The damaged F-4 was "powered off" in the sense the engines have been shut down and "serviced" the way it has been moved far enough (142 km).
This looks quite close to that has been asked.

The concept of Airborne Aircraft Carrier has been proposed in the past. Beginning with Airship. There were many attempts to launch and recover small aircraft between 1933-1935.
Boeing 747-AAC (Airborne Aircraft Carrier) was proposed to launch and recover up to 10 microfighters. It never went to reality.

It sounds like in each of those cases, the docked aircraft would be substantially smaller than the AAC?


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The feasibility of using wind turbines to construct a giant quadcopter
I'm not looking for a definitive yes/no answer here so much as an illustration of the calculation with some fairly reasonable assumptions and references to relevant and useful data.
My assumptions are:

It's possible to modify the generator in the wind turbine so that it can be operated as a motor,
The motor efficiency would be the same as the generator's efficiency,
Power into the motor would be the same as the turbine's rated output power,
The wind turbine can operate while its shaft is oriented upwards, in the zenith-pointing direction, and
Power will be supplied from the ground via a high-voltage line.

I imagine this enormous quadcopter being used only during fair weather as an amusement ride or attraction, similar to a tethered helium balloon. There might be other applications in, for example, construction.

I'm specifically interested in understanding whether a modern wind turbine and its blades are already engineered to be light enough and powerful enough for a limited-use VTOL aircraft.

The lithium ion battery fire would be epic.

Motors and generators are essentially the same thing, there may be some modifications required, but it would not substantially change size, weight, power, etc.
The motoring efficiency would be very similar to the generating efficiency.
The motoring power would be very similar to the generating power.
Unknown consequences of changing the orientation.  Wind turbines have a thrust bearing, but it is incorrectly oriented (push vs. pull).  The blade & hub is not designed for the loads in that direction, it would be interesting to see how the overall loads changed.
Power from the ground, OK.

Problems:

Quadcopters rely on rapid changes in RPM to modulate thrust for control.  Wind turbines are enormous and have enormous inertia.  Rapidly changing RPM would require a far larger motor than a turbine's generator.  There is generally considered to be an upper limit on how big a rotor can be and still use RPM control -- while we can discuss the exact value of that limit, it is much smaller than a modern wind turbine.
Wind turbines have variable pitch blades.  You might conceive of using blade pitch instead of RPM to control the vehicle.  However, wind turbines are designed to change blade pitch very slowly, it would require a major re-design of the pitch control mechanism to achieve the fast rate control required to fly an aircraft.  That mechanism would be larger and heavier.
Wind turbine blades are designed to extract power from the air with great efficiency -- i.e. to be driven.  They are not designed to efficiently produce thrust.  A windmilling rotor operates in a fundamentally different flow regime than a thrusting rotor.  Although they look similar, the design of a windmill rotor is fundamentally different from a propeller.  Consequently, propellers make terrible windmills and windmills make terrible propellers.

That last point is really interesting. Do you remember from where you learned that?

@phil1008 https://aviation.stackexchange.com/q/8851/31936

@phil1008 When we talk about an an actuator disk, we describe the flow in different states.  Typically we call them propeller, windmill, turbulent wake, vortex ring, and propeller brake - though some authors use different terms depending on whether they come from the propeller, helicopter, or wind turbine community.  The differences are determined by things like whether thrust is positive or negative, power is positive or negative, and flow acceleration is positive or negative.  In some of these situations (e.g. vortex ring state), simple momentum theory does not work.


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What are the defining characteristics of aviator glasses?
The title pretty much says it all.
It's easy to find pages for resellers of aviator glasses or aviator sunglasses, but what are the defining characteristics of these as compared to ordinary glasses or sunglasses?
If one were to buy from an ordinary optician not specializing in aviator customers, what should one look for or ask for to get "aviator" glasses? Correspondingly, what should be avoided?

Possible duplicate of What kind of sunglasses are good for flying?

Polarizing glasses should be avoided, as many digital instruments have liquid-crystal displays...

The term "aviator sunglasses" really is more about a style than specific aviation features. My recommendation would be to find non-polarized sunglasses that you like, "aviator" or not. You should also try them on with your headset so you see how it fits.

Aviators are nothing more than a marketing gimmick developed by Bausch & Lomb back in the 1930s.  The Wikipedia page on them quotes:

They are characterised by dark, often reflective lenses having an area
  two or three times the area of the eyeball, and very thin metal frames
  with double or triple bridge (so-called ″bullet hole″) and bayonet
  earpieces or flexible cable temples that hook behind the ears. The
  original design featured G-15 tempered glass lenses, transmitting 15%
  of incoming light. The large lenses are not flat but slightly convex.
  The design attempts to cover the entire range of the human eye and
  prevent as much light as possible from entering the eye from any
  angle.

It’s possible that Bausch & Lomb referred to the style of sunglasses as aviators given the popularity of early aviation and barnstorming during the late 20s and early 30s. The brand really acquired notoriety and popularity during World War II when they were used by military personnel and began to get associated with flying. General Douglas MacArthur often wore them.  Postwar, the style became popular with a number of celebrities as well.

I flat out cannot believe I didn't find that Wikipedia page when searching. Thank you.


text:
Loading exerted by propeller blade
I have a query on where propeller blade loading points.
On this paper I found this picture:

In that paper, they say that, if we define a vector $\vec{R}$ from source (blade section) to observer, and a unit vector $\vec{n}$ aligned with the local force exerted by the blade on the fluid, the dot product $\vec{R} \cdot \vec{n}$ will be maximum when "the net loading exerted by the blade on the air points towards the observer".
Specifically, they say that $\vec{R} \cdot \vec{n}$ has a "maximum magnitude
when the blade is moving towards the observer". This occurs about at $n=10$ in the above picture.
What I'm wondering is: how is it possible that at $n=10$ (or a bit later than that) the loading is pointing in the direction of the observer? I mean, at $n=10$ the blade is showing its pressure surface to the observer and we know that the loading exerted by the blade on the fluid points away from the suction surface, instead, which is on the other side.
Thank you for your help.

If it's a turboprop propeller it's a bit simpler. If it's a piston engine, you end up with a myriad of loadings beyond aerodynamic and centrifugal, the power pulse and inertial reaction every 180 deg on a 4cyl engine, made more complex by resonant flex of the blades in response to those pulses. A metal prop is a big tuning fork. It's a mess.

Why does the location of the observer matter? Is the observer in-plane with the disc, or at some distance in front of or behind the plane of the spinning blades?

The paper is behind a paywall so I cannot access it. But as the title of the paper clearly states, it addresses the noise of a contra rotating propeller so that the term "loading" might be related to noise loading rather than aerodynamic loading.

Hi all, thanks for your replies.
The "loading" the paper refers too is the loading exerted by a blade on the air. It is essentially the pressure force, which is normal to the blade surface.
The observer is in the propeller plane to the right of the propeller. The observer location matters because the R vector is defined from a blade element to the observer.


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How to calculate fuel burn rate in Model BK117 helicopter
I am confused which parameters we are use, I have fuel quantity, fuel flow, fuel weight and speed etc is there so how to using and which parameters using the Endurance time calculate in helicopter

One thing I don't understand is why you would need to calculate these for that specific helicopter. You can get these values much more accurately from a Google search

@AadirajAnil these questions are normally related to some homework problem...

Given the fuel quantity and the fuel flow, it should be quite trivial to calculate endurance. Consider the following:
Fuel flow simply states the amount of fuel consumed during a given timespan. For example: $\dot{f} = 5 \frac{l}{min}$. Together with a given fuel quantity e.g. $f_{total} = 697 l$ we can now calculate the time until this fuel quantity is consumed:
$t_{endurance} = \frac{f_{total}}{\dot{f}} = \frac{697 l}{5 \frac{l}{min}} = 139 min$
It should be noted that these are approximate values, I do not claim that these values represent the actual aircraft examples exactly.


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IAF is part of the hold at an uncontrolled aerodrome. Is the racetrack procedure necessary?
I am studying instrument approaches and I am not quite sure about this particular NDB-based approach procedure. Particularly, in regards to two things that, in my head, are conflicting:

"Direct entry to the procedure is mandatory", to me, the "procedure" seems to begin at the 009º QDR course as you pass overhead the locator, as another way of saying that you can't just get there at an absurd angle and try to intercept the 009º QDR after coming in on like a 270º heading.

However, this seems to me like you could ignore the racetrack procedure entirely provided you are approaching on the specified entry sector. Because if the "procedure" mentioned in "Direct entry to the procedure is mandatory" also includes the racetrack, doesn't that mean that you cannot even ENTER the racetrack itself in a teardrop or parallel entry?

I keep reading "IAF" as "Israeli Air Force"...

Perhaps this is a case where EASA regs differ from US? For an approach in the US, if your turn at the IAF is 90 deg or less you may (if so instructed by ATC) turn and continue straight in rather than fly the PT. Likewise if vectored onto the approach. If you are not so instructed you must fly the PT. In practice, my understanding is that ATC sometimes forgets to say "straight in" and pilots do it anyway and no one cares.

@FosterBoondoggle that is true if being vectored to final, but in this situation the full PT is being flown. See my example below of a holding pattern used for alignment in the US.

#1. Yes, you must fly to the NDB directly. If you are coming from the south (inbound heading between 339 degrees and 039 degrees), you enter the procedure turn outbound. If you are inbound from any other direction, you must request one turn in holding for alignment.
#2. The holding pattern is not part of the procedure. If you need to use it for alignment, you must first request that from ATC (see the note in the lower left section of the chart).

One of my colleagues, who just did his IFR checkride, passed, but said the examiner reprimanded him for coming in from a direct entry but not performing the racetrack. If the holding pattern is not part of the procedure, how can this be the case?

From which bearing was your colleague approaching the NDB? Also, a holding pattern that is not aligned with final is either an arrival hold (e.g., for delaying) or for alignment. That is separate from a procedural course reversal.

He's adamant that he was doing a 030 heading inbound the NDB.

@GapeN , your colleague was correct to cross the NDB and join the 009° outbound. No turn in holding is required for that.

You do not need to fly a procedure turn if (I use SHARP TT).
Straight in approach clearance,
Hold in lieu of procedure turn,
DME Arc,
Radar vectors to final,
No Procedure turn published on chart,
Timed approach from a hold fix,
Teardrop entry course reversal.
So, that being said your #1 is correct. You want to be doing a procedure turn to intercept for final approach course smoothly. #2 while it may seem easier for you to just ignore and make a decent turn to get on course it is important to be as predictable as possible to ATC (or in uncontrolled, other aircraft) along with this PT side of the radial being protected from other traffic at that alt. So if this PT is published and you do some other sort of PT you are risking hitting other traffic or making ATC go crazy by surprising them with an unpublished hold.
I live in Southern California (USA) and holding on the correct side of a radial is huge. It could mean life or death due to the high volume of traffic. I have held over a VOR with 3 others at different altitudes and having to deal with traffic using the VOR as an airway. Hopefully this helps on why its published like that and why its so important!

By "PT", do you mean the procedure turn represented by the racetrack at the IAF, or the 45/180 procedure turn after the spending 2:30m on the 009º QDR?

Yes PT is procedure turn its commonly depicted at PT on US charts. So what they want you do do exactly is you’ll fly over the fix (the NDB) perform the procedure turn by properly entering depending on your sector. Ex. Sector 3 is direct entry (according to the chart this is the entry sector they want you to be doing and to do 2 or 1 you must request it), sector 2 is teardrop and sector 1 is parallel entry. Then you’ll do the racetrack pattern with the 009  inbound and 189 outbound. Upon the final 009 inbound you will overfly the fix continue outbound on 009. Usually this will be 1min -1.5mins

Then turn to heading 324 for a min or so, then finally you will fly inbound on the QDM 189 to land. They are saying direct entry to the PT is mandatory assuming your flying the PT and not being vectored to final.

@TaylorB, be careful not to confuse a hold in lieu of procedure turn with a racetrack designed for alignment. What is true for you in SoCal is not the same internationally. The closest approximation would be a charted arrival hold that you request to use for alignment outbound. An arrival hold is not part of the procedure.

See this example: https://aeronav.faa.gov/d-tpp/2401/00903TY24R.PDF

In this case there is no such thing as "being vectored to final" because Evora is uncontrolled and has no radar service.


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What is the relationship between the wingspan and the electrical power of the DC motor?
I'm working with a cargo plane and I'm trying to find out if it's more efficient to work with more wingspan or more engine power. For example, having a variation of 300 mm (I have a model airplane), or 100 W of engine output power. How much more gain in MTOW and takeoff will I have in each case? How can I start thinking?

Working with more engine power is definitely not more efficient since you need more energy to get that power

I think you need empirical data. It may depends on too many factors, and we cannot scale aerodynamic data from airplanes to models (geometry rules forbid it to us).

Electrical implies (but not guarantees) battery power. Battery weight is a HUGE factor - add 100W of power but don't increase the battery size then your payload capacity and/or speed goes up but your range goes down. Increase the battery size and your payload capacity goes down. A lot of interacting factors.

Are you adding 300 mm to a wing that's starting at 100 mm, or is already 10 meters wide? Are you starting with 100w of power & doubling that, or with 1,000w and adding 10%? Without this most basic info, there's no conceivable way to evaluate trading off between those options. (Even then, the "everything else" involved will make realistic analysis pretty hard until far more details are given.)

You can start by showing your work.  Early aircraft with limited power favored more wing.  You'll get your weight off the ground but will be very slow in the air.  Structural strength is also important.  Very high AR wings must be strong enough.  Desired cruising speed and reserve power for climb are also important considerations.

Adding the data as a complement, I have an aircraft with a wingspan of 1.1m and I would like to know if increasing 100W (I currently have a 700W engine) of engine power would be more effective than increasing 300mm of wingspan. And if so, how is the thought/bibliography for the answer?

What is the weight of your aircraft?

"I would like to know if increasing 100W of engine power would be more effective than increasing 300mm of wingspan". More effective in terms of what? Endurance? Range? Maximum speed? Stall speed? Turn radius? Manoeuvrability? Vertical speed? ...

If you think you've got enough additional info to warrant the question being reopened, please [edit] the question and add the info there. If it's buried in the comments, odds are good that people won't see it

Glide tests with the model in its current configuration and weight will answer a lot of questions about power required.
This depends a lot on your weight.  The heavier the aircraft, the higher its best glide speed will be.
Larger wings give a lower glide speed for a given weight.  This translates to less power required.
Smaller wings and a larger motor enable higher cruise speed.
As you already have 700W, which is almost 1 horsepower, and a 1.1 meter wingspan, you might actually be a bit overpowered.
As models generally fly in the 10 - 40 knot range, you can probably add more wing.  Slower flight and landings will be easier to control as well.
One other thing to consider regarding lifting efficiency, especially on this scale, is Reynolds number.  Lift/Drag ratios greatly increase when Reynolds number reach higher values.  This is accomplished by increasing speed.

Reynolds number = Velocity m/s × chord m/(1.46 × 10$^-5$ m$^2$/s)

The effects of Reynolds number on your wing efficiency can be found at airfoiltools.com.

Lengthening wings to increase the aspect ratio (AR) is usually the better way to go if you want to increase efficiency, but you need to make sure that you're using something well-designed to support the wing.
Things to consider while designing wings is to watch out for flutter, and to make sure it doesn't flex too much. You should also be considering the shape and airfoil of the wing itself. This will increase efficiency further.
I'd caution against the motor because it doesn't increase efficiency, while increasing weight and battery draw.
For


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Is it possible for someone to buy a fighter jet?
Yesterday I came across a video of a person who purchased 3 Harriers ("Harrier Jump Jet").
This brought up the question of: assuming he/she has the capability of paying for the price, is it possible for a regular civilian to purchase a fighter jet?
The video did note that the person was a retired test pilot. Is that a reason why he was able to purchase the jet?

related Which 2-seat 4th generation fighter/engine/configuration (de-militarized) would the best supersonic long-range civilian private transport?

Highly related: Can a civilian purchase a new (Not Retired) fighter jet?

This is like a trading question. Purchasing is the easiest part because there are plenty of sellers with surplus military equipment. Your first difficulty could be payment, if your bank's jurisdiction regulates arms purchase. Importing it to your home county is yet another challenge, e.g. to clear the customs. And then finally it's the permit to operate, subject to aviation regulations. However, if you pick a jet trainer instead of a fighter, all of these would be a lot easier, and performance is similar.

Just type "jet fighters for sale" into your search engine, or look at the "warbirds" section of Trade-A-Plane - where you'll find 3 Harriers listed (price not shown), as well as a MiG-29 for $4.6 million, a MiG-21, MiG-17...


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How to get the true altitude in non-standard temperature using a regular barometric altimeter?
I wondering how can we get the true altitude with non standard temperature on regular barometer altimeter, as we know that if we set Kollsman window to the current local setting the altimeter SHOULD shows the actual altitude above sea level ( True Altitude ), but if the temperature is above or below standard the indicated altitude on the altimeter will not be the same as the actual true altitude.


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Can ATC issue instructions to exclude certain aircraft over international waters?
As an example, the Anchorage Air Route Traffic Control Center's sector extends all the way to the North Pole, far beyond 200 nm of the US coastline.  Suppose that the US decides to "close its airspace" to Russian aircraft…
Would Russian aircraft still be allowed to fly in Anchorage's sector near the North Pole, which is in international waters?
Can ATC issue instructions to tell Russian aircraft to turn around?  If so, by what reasoning?

Closely related.

Sure, US ATC can tell Russian aircraft that they're not allowed. Countries coordinate traffic outside their borders all the time. (Mostly over the ocean of course, but here's a video of a Canadian pilot, flying in Canada, who had to get permission from a US ATC facility to fly where he wanted.)
When in (or over) international waters, any vessel (or airplane) is considered to be under the laws of the country in which it is registered. Member nations of the ICAO have treaties with each other saying that airplanes will obey foreign ATC when flying in their jurisdiction, even if that jurisdiction extends beyond that country's borders. So, if a Russian airplane tries to fly through US-controlled airspace, the US says no, and they do it anyway, then, at least technically speaking, they're violating Russian law and can be prosecuted in Russian court for it (or however they handle aviation violations over there).
Of course, with the current tension between the US and Russia, Russian law enforcement are going to be a lot less likely to go after their own citizens because the US complained about them. The other side of that coin is that such planes are (slightly) more likely to be intercepted by US fighters, and the pilots of those fighters are going to be far more keyed up and ready to shoot if the violating plane doesn't comply with their instructions.

"and the pilots of those fighters are going to be far more keyed up and ready to shoot if the violating plane doesn't comply with their instructions" - If it's a civilian plane?  Highly unlikely, barring evidence of an actual hijacking.

@Vikki I never said they would open fire for no reason. But they are going to be much more tense and on alert then otherwise.


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How are these Airbus A350s flying in formation without being affected by wake turbulence?
The first photo in this article appears to depict three Airbus A350s flying closely in tandem at approximately the same altitude.
I thought that a large jetliner would leave a trail of wake turbulence behind it that would be unsafe for several minutes.  Is there something special about the Airbus A350 that makes this formation possible?  Am I misinterpreting the photo?
The V formation in the second photo seems much more reasonable.

Probably because of couple of factors. 1) they are flying slow. 2) they look close in the pictures but they are actually quite far from each other.

@Tasos Flying slower makes stronger wake turbulence. This is why the worst case for wake turbulence is on and near runways (where planes are flying much slower than normal.)

@reirab i didnt know that. is just that i saw a few videos and the planes looked like they were flying slow. like this one https://www.youtube.com/watch?v=jdrZdmRqmyU

@Tasos That's probably more of an illusion due to their size and altitude than anything else. They're probably at cruise speed. Even near the ground, the shear size of a jet airliner can make it appear to be flying much slower than it actually is.

You are right that wake turbulence would be a danger when flying behind another aircraft like that.
The perspective of the photo makes it hard to see the actual position of the trailing aircraft relative to the ones in front of it. Pilots do this sort of thing in airshows when they fly in formation. The trailing aircraft are generally slightly below the ones in front of it. This allows good visibility of the plane they are following. At the higher speeds that these jets are flying at, the wake vortex will not drop enough to impact an aircraft fairly close behind and below.
See this old photo of the Thunderbirds. From this perspective, it is easier to see that the trailing aircraft is also below the flight path of the leading aircraft.

From Farhan's answer it seems that the wing tip vortex travels downwards (1st image). Why would trailing aircraft then fly below the the ones in front of it to avoid the vortex? Edit: Saw your edit, thank you for the fast response.

@BartArondson I edited to clarify that point, let me know if it didn't answer your question.

The reason the following aircraft flies below the leader is to put it into an inertial flightpath that would prevent a collision. Imagine you're behind and below the airplane in front of you, and you pull back on the yoke and point the nose at the plane in front: because you're trading altitude for airspeed, you'll safely pass behind the airplane in front of you. If you're above the lead aircraft, and you push down to put your nose on the leader, you'll speed up and hit the aircraft in front.

Yes, it does appear that the airplanes behind the lead in a formation would be affected by wake turbulence. In reality, there are several factors which contribute to make it not so risky.

Airplanes make the formation on high altitudes. Wake Turbulence becomes a bigger issue during takeoff and landing phases. As mentioned on Wikipedia:

Wake turbulence is especially hazardous in the region behind an
  aircraft in the takeoff or landing phases of flight. During take-off
  and landing, aircraft operate at high angle of attack. This flight
  attitude maximizes the formation of strong vortices. In the vicinity
  of an airport there can be multiple aircraft, all operating at low
  speed and low height, and this provides extra risk of wake turbulence
  with reduced height from which to recover from any upset.

Wake turbulence travels like this (source):

As you can see, based on weather and wind, the following airplanes would know what should be their best position in reference to the lead. They avoid this situation (source):

Airplanes in formation do not fly on the same altitude but are either slightly above or below the lead. This is very hard to figure out when you are watching from ground, or in several pictures.

Just curious: what is the source of the two images?

@BartArondson I found on related articles on Internet.

The Wikipedia quote is slightly misleading. The wake turbulence is stronger behind aircraft flying slower independent of altitude; it only so happens that at low altitudes aircraft usually fly more slowly. Also while it is more dangerous near the ground (also because there is limited manoeuvring space to recover), it can still be pretty violent at cruise level.

Can't we stop perpetuating this wingtip wake myth? It's the rolling up of the vortex sheet behind the wing, and those graphics don't do it justice at all.

@PeterKämpf If you'd like to set the record straight, I'd love a full explanation on my question here:  http://aviation.stackexchange.com/questions/8877/how-does-a-wing-form-wake-turbulence

What I'm going to say is my own opinion. I don't claim it's true, that's just my understanding of wake turbulences... I'm eager to learn, so feel free to correct me if I'm wrong :)
I'm assuming wake turbulence is moving air (airflow) caused by an aircraft flying ahead, wether airflow moving down, up, sideways or spiralling - I understand the pressure difference - Without going deep in particles motion (air molecules) I think you just have to consider four things :

your position behind a the leading aircraft : right behind its fuselage, right behind one of its wing tip, beyound its wingtip trail, from the axis of the leading aircraft.
your distance behind the leading aircraft : a few feet just behind (without colliding with) or miles away ?
wake size increases as you go far behind the aircraft producing the wake turbulence, but its strength decreases accordingly.

The point 3 tells you where NOT to position your aircraft behind another. Especially at a (relative) location where the wake is still powerfull, and its size is large enough to affect your controls surfaces and wing envelope, and send your aircraft in an uncomfortable motion, brutal gain or lost of lift, differential lift, etc.
So, right behind another aircraft, you can fly without been affected (much) by the wake turbulence, because the wake size is not large enough to send your aircraft sideways, however, you still encounters slight turbulences, especially if you fly just right behind the wingtips. Worst situation is when you fly several hundreds feets behind the aircraft : large volume of strong spiralling airflow that you can't visualize. Miles aways, the airflow velocity is assumed to have dissipate enough because of its lost of strength due to expansion and air resistance.
Of course, the wake turbulence of a Cessna is not the same of an A380. Weaker and smaller for a Cessna, that dissipates pretty quickly. This leads me to introduce point four :

The size of the plane creating the wake, and the size of the following plane. Surprisingly, you can put an A350 right behind another A350, but you can't put a Cessna right behind an A350. This, because of the three points above : The Cessna will likely be affected by the slightest amount of airflow while the A350 wont much, because of surfaces size (wings/ailerons) The wake (especially the wingtip vortex) hasen't grow enough to hit the following A350 hard enough.

However, that doesn't mean there aren't risks. Main risk in such close formation is vibration. Test pilots are trained to identify their origin and deal with.
And also, I don't have a comparable aircraft in size, so I'll take a DC10, or a B52 : I think you can't put such earlier generation of aircraft right behind an A350 (or 787) because of Wing technology (this related to the question "does the A350 has something special ?") Old aircraft have much less flexible wings, that makes them really sensible to brutal airflow. I'm not a physicist, but more flexible wings better distribute the load factors along said wing, considerably reducing vibrations and fatigue.
That's why wake turbulences are more dangerous at lower speed : the wake can grow enough to compromise the flight enveloppe of the following aircraft even at the tightest separation. That's why wake turbulence are a serious issue at lower altitudes where the moving airflow is really dense and affect surfaces greatly. Logically, wide wakes should dissipate more quickly in dense atmosphere, but the (tiny) spiralling vortex can last several minutes and is still very dangerous especially for smaller aircraft.
Fighter jets in airshow and flight formation :
 a) They fly at high speed => not enough expansion to affect nearby followers
 b) tandem formation requires lining up with leading aircraft => good distribution in differential lift.
 c) in tandem formation, followers are usually below (rarely above) the preceding aircraft.
Finally, a Cessna control surfaces (and flaps) are more sensible than the ones that equip a 777. Never get a Cessna below and behind a 777 in a five to ten minutes timespan unless you know where the wake turbulence path has drifted and/or landed.
Side note : I'm not a pilot. I'm just interrested in how an aircraft flies. There are tons of documents out there about wake turbulences, wake vortices, stall schemas, etc. Some are contradictory, some are complementary. At a molecule level, the magic of an airflow looks like a perfect example of the theory of chaos. But the above is the summary of what I found here and there. That's no science, just opinion.

"That's no science, just opinion"


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Why would pilots get special cups?
This Reddit post shows a picture of a disposable cup that is labeled specifically for use by pilots only:

Is that seriously a thing? If so, why? There's some explanation in that discussion thread about fitting cup holders more tightly, but it doesn't sound like a plausible explanation to me, since the cup holder should be designed to hold the cup properly, not the other way around.

Sterilized to maintain a sterile cockpit? :)

The answer is pretty simple...
It is because those cups have matching lids that won't fit the passenger cups. Lids are used on the flight deck to keep the liquid inside from spilling on the (ever increasing) electronics, especially those located in the center console between the pilots (which house the radios in a lot of airplanes).
So some airlines have made a policy of any cup going into the cockpit must have a lid, and these cups specifically have lids.

See https://en.wikipedia.org/wiki/Fate_Is_the_Hunter_(film)  from 1964

I think there's something missing from your answer: I get that lids prevent spillages, but why would mismatched lids help? A passenger cup with a fitting lid would suffice, so why use different ones for the pilots?

@RogerLipscombe Because the passenger cups don't have kids at all... Yes they could just get lids that fit the passenger cups, but the warning also reminds the cabin crew that this cup must have a lid.

@RogerLipscombe a more appropriate question would be "why have cups of different sizes for crew and passengers". That could in fact be an interesting one.

@jwenting surely that is to accomodate a bracing draft of brandy in the coffee...


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In a hydrogen fueled plane, could part of the fuel load be allowed to become lighter than air to offset or trim for the load?
This is inspired by How can adverse maneuverability due to a fuel tank behind the rear pressure bulkhead be mitigated in Airbus ZeroE hydrogen plane? .
I was thinking, with the cryogenically stored hydrogen (and required equipment) being heavy and so far away from the centre of lift of the craft in Airbus's design, it might be possible to trim the aircraft by releasing some of the hydrogen into a chamber where it can be lighter than air, reducing the nett weight of the tail, rather than trimming aerodynamically or with a movable weight.
I'm not good enough to do the math, but I suspect the size of the chamber would have to be colossal to make enough of a difference to matter - with the added bonus of being essentially an explosion waiting to happen.
Would there be a sweet spot for the average temperature of the stored hydrogen that balances the weight (including weight of corresponding equipment) against the volume of fuel stored, or is it going to be a case of coldest/most dense is the most efficient and trim needs to occur the traditional way?

I'm not sure this platform is meant for "please do the maths for me". It would be  courteous to include your attempts at the problem and ask for help when you get stuck.

@Sanchises I think this is reasonable question to ask that could maybe be answered by intuition by those experienced without diving deep into mathematics. At least an opinion if such a design is at all possible

While pressurized Hydrogen can be stored relatively safely, low-pressure hydrogen, like that used to make an aircraft buoyant, is dangerous.

@zymhan: Low pressure hydrogen is not dangerous itself.  What's dangerous is having hydrogen mix with air.

Thanks @Sanchises, I'm not after the math itself, if someone wrote out an equation, I'll be honest, I probably wouldn't even understand half of it. As Erik mentioned, I'm more after testing my intuition - I am thinking that the space to expand the gas into would have to be a size so large that it would no longer fit into a practical aeroplane shape.

The buoyancy of hydrogen is not as high as expected, and is even lower at high altitude due to the lower ratio of the density of hydrogen and air.
Just for comparison:
The tail tank of the A380 has a capacity of 23698 l.
When this tank is filled with kerosene / air / hydrogen, the content weights:
Substance           | Density [kg/l] | Weight of 23698 l [kg]
--------------------+----------------+-- -----------------
Kerosene            | 0.8            | 18958
Air (sealevel)      | 0.00122        | 28.91
Air (10km)          | 0.00041        | 9.72
Hydrogen (sealevel) | 0.00009        | 2.13
Hydrogen (10km)     | 0.00003        | 0.72

At sealevel, the lift of this tank filled with hydrogen is 28.91kg-2.13kg=26.78kg.
And at altitude, it's just 9.72kg-0.72kg=9.00kg
This is nothing compared to the 18958kg of kerosene, and shows that it makes no sense to fill any volume in an aircraft with hydrogen for trim.

Thanks! Really good point about the reduced ratio between density in the tank and outside air as altitude increases, it hadn't crossed my mind but in hindsight it makes perfect sense.

It is customary to pump liquid fuel for this purpose, as a given trim change requires far less volume than moving gas around.
Exchanging the actual gas requires large volumes, and also some way to keep the hydrogen and air separate as their relative volumes change. Airships typically have an inner gasbag called a ballonet for this purpose. In the case of an airship, gas is pumped in or out of the surrounding gasbag. The ballonet is vented to the air and shrinks or expands to maintain constant pressure and prevent the outer dimensions of the gasbag from changing.
If you created enough volume in a plane, it would make it noticeably larger and that would add both weight and drag. Filling large containers with hydrogen or any other fuel vapour would be as unacceptably dangerous as it is for all airships, so you would have to use a separate supply of inert lifting gas such as helium.


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How is a stolen aircraft returned to its owner?
Once the authorities figure out where a stolen GA aircraft is, do they get the owner to come take it back? do they fly it back to its home airport? How exactly does the owner get his hands back on it?

Simplest seems just to say "hey, your aircraft is standing there, come and pick".

Probably the same way a car is returned to its owner. Cops don’t drive it back to its home. It is impounded. If it had been used in a crime, it is kept as evidence until after all evidence is gathered, recorded, and filed. If they can not move the stolen item to an official impound yard or lot, it will be impounded in place. Locking it in a hangar and placing devices on the landing gear or propeller would deter a subsequent theft. They would then notify the owner or the insurance company (who is the new owner after paying the claim) to retrieve the item.

@DeanF. Please provide an answer in answers, not in comments.

@KorvinStarmast As Dean's comment is only a guess, it is suited for a comment. Answers should meet some quality standard (defined in the help center) and if Deans thinks its comment does not meet those quality standard, putting it in comment can help other writing good answer.

@KorvinStarmast - I have no idea how an airplane is returned. I do know how a car is returned in Texas. Even then, it may vary with police departments. A small-town country sheriff may spare the man-power to return the car in person to a resident who they know personally. But, larger departments will not spare the man-power or assume the liability. Chain of custody issues and all. I doubt they have a pilot on staff who would be willing to assume that responsibility, either. The OP can extrapolate an answer about airplane returns from there.

If the owner retains ownership (rather than an insurance company who paid out a claim on the loss), it's up to the owner. The owner can either go pick it up if it is airworthy, or arrange for someone to ferry it back to the owner's home airport.


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How much altitude can a glider gain only from losing speed?
Say we have a glider flying near its top speed. If pilot decides to "trade speed for altitude" how much can it climb before hitting stall speed? Assuming pilot will use most effective climb rate for the task.
I know it depends on many factors, but what kind of figures are realistic?
Reverse situation - how much altitude glider needs to gain speed (say its top speed)?

Can't give you anything like an exact answer, but enough to do a loop.

Yeah I would say you can gain somewhere around 500 ft+.

Basically you are asking how high a glider can realistically be expected to pull up after a low-altitude high speed pass that approaches Vne.  One answer derived from real-world observations would be high enough for a downwind-base-final circuit followed by a landing...

You can calculate a theoretical maximum (which will be well above the actual practical height gain) by considering the conversion of kinetic to potential energy. You can also make estimates based on observing a glider performing a loop.
Calculate the kinetic energy of the glider at maximum speed as mv2/2, where m is the glider mass and v is the maximum level velocity. Then do the same for the stall speed mu2/2. Take the difference. This is the kinetic energy lost in the maneuver.
Assume this is all converted to potential energy, which is given by mgh, where m is the mass, g is the gravitational constant and h is the height gained. Rearranging this (and cancelling m) gives us

h = (v2 - u2) / 2g

Again, the real number will be much less than this, because of drag during the maneuver, but it will give you a ballpark.
According to Wikipedia, a Schleicher ASK 21 has a stall of 40 mph (18 m/s) and a top speed of 170 mph (76 m/s), which seems to be one of the larger ranges among common (and non-performance?) gliders. With g=9.8m/s^2, and calculating to two digits of precision, that's a theoretical max of h=280m or 920 feet. (Thanks K A Buhr)
When a glider performs a loop maneuver, it does so by diving to achieve a high speed, pulling up to a low speed (upside down) and then diving again. This is somewhat analogous to what you are describing, although gliders don't reach their theoretical maximum speed in the maneuver, and they may in fact go under their stall speed at the top. An examination of performance gliders looping show that they can gain several hundred feet during a maneuver.

+1, but this would still be better with a numeric example from a typical sailplane. And I don't think the real height gain would be much less than the upper bound you get from vis-viva - would expect it to be at least half of the theoretical max.

According to Wikipedia, a Schleicher ASK 21 has a stall of 40 mph (18 m/s) and a top speed of 170 mph (76 m/s), which seems to be one of the larger ranges among common (and non-performance?) gliders.  With g=9.8m/s^2, and calculating to two digits of precision, that's a theoretical max of h=280m or 920 feet.  (Feel free to incorporate into the answer, if the math and unit conversions look right.)